A Harvard think tank concludes in a study being released today that despite a multibillion-dollar price tag, the 2002 state constitutional amendment to reduce class sizes has had little impact on student test scores.

But it'll hardly be the last word.

As Florida voters gear up for another amendment on class size, this one to make the existing amendment more flexible, the debate is heating up over two complicated questions.

Is it working?

Is it worth it?

"Cost and performance," said Darryl Paulson, a retired political science professor from the University of South Florida St. Petersburg. "The 10,000-pound gorilla in the room is going to be cost factors, and are voters willing to pony up all the additional money if they're not convinced that there's tangible benefits."

The class-size amendment mandates that all classes in core academic subjects be reduced to no more than 18 students in grades K-3, 22 students in 4-8, and 25 students in 9-12.

After spending $15.8 billion, the state has shrunk class sizes enough to meet the caps as a schoolwide average. But this fall, the caps must be met in every classroom.

The proposed amendment, put on the ballot by a Republican-led Legislature, would freeze the amendment at the school level. It would also prohibit classrooms from being more than three students over current limits in pre-kindergarten through third grade, or more than five students over in other grades.

The potential savings are unclear. But it's likely to be tens of millions of dollars each year — and possibly more.

The new study is the first to look at the effects of class-size reduction in Florida.

Using data from the Florida Department of Education, Harvard research fellow Matt Chingos looked at the impact of smaller class sizes on grades 4-8 through 2007. His conclusion: The amount Florida spent on reducing class sizes had no greater or lesser effect on academic performance than allowing schools to spend similar pots of money as they see fit.

USF education professor Sherman Dorn, who reviewed the paper at the St. Petersburg Times' request, said it was a clever attempt to tease out the effects of class-size reduction from other factors affecting student achievement. But it's not the slam dunk some class-size amendment critics may hope it is.

"People on both sides of the policy issue can probably cherry-pick information out of here," he said.

Florida students have made some of the biggest gains in the country over the past decade, according to national reading and math scores. But it's tough trying to determine how much credit should go to smaller classes vs. other big changes, such as school grades and an intense focus on reading in early grades.

In coming months, both sides will give it their best shots.

Florida Education Commissioner Eric J. Smith, who supports the proposed amendment, has not seen the study. But he noted its findings seem to jibe with reams of research that conclude modest reductions in class size yield, at best, modest gains.

"The research has been consistent in that respect," he said.

Damien Filer, former spokesman for Florida's Coalition to Reduce Class Size, the group that led the 2002 effort, said parents will make up their minds regardless of what dueling studies say.

"Is it worth some investment for their child to be in a class where they can get adequate attention from their teachers?" Filer asked. "They'll say yes every time."

Ron Matus can be reached at matus@sptimes.com or (727) 893-8873. Jeffrey S. Solochek can be reached at solochek@sptimes.com or (813) 909-4614.